1. Field of the Invention
The invention relates to a steering system for a vehicle. More particularly, the invention relates to a steer-by-wire steering system for a vehicle.
2. Description of Related Art
In recent years, development of a steer-by-wire system (hereinafter, SBW system) in which a steering unit including a steering wheel and a turning unit including a turning mechanism are mechanically separated from each other has been proceeding. The SBW system may include a backup mechanism including a clutch that mechanically couples the steering unit to the turning unit for the purpose of fail-safe. In the thus configured SBW system, the turning mechanism is controlled by an electric signal in response to operation of the steering wheel (hereinafter, SBW control) when the system is normal. On the other hand, when there occurs an abnormality in the SBW system, the clutch is quickly connected, SBW control is stopped, and the system is switched into assist control for reducing a burden on driver's steering.
However, if the clutch is connected although there is no abnormality in the SBW system, the steering wheel may rotate as a result of continuation of SBW control (in other words, operation of a turning actuator). In SBW control, the turning actuator is driven such that an actual steered angle of steered wheels follows a command steered angle. When the clutch is connected, the steering wheel and the turning mechanism are mechanically coupled to each other by the clutch. Therefore, when the turning actuator turns the steered wheels in SBW control, the steering wheel mechanically rotates in interlocking with the turn of the steered wheels, so the command steered angle changes (hereinafter, the phenomenon that the steering wheel is rotated through this operation may be simply described as that the steering wheel is rotated interlockingly). Therefore, the deviation between the command steered angle and the actual steered angle does not reduce, and self-steer may occur.
Japanese Patent Application Publication No. 2007-137294 (JP 2007-137294 A) describes a technique switching from SBW control to assist control when an SBW system determines that a clutch is connected in process of SBW control.
According to JP 2007-137294 A, because SBW control is executed until connection of the clutch is detected, there is an undeniable possibility that the steering wheel is rotated interlockingly in a period from when the clutch is connected to when SBW control is switched to assist control.